Recently, HCCI (Homogeneous-Charge Compression Ignition) combustion in which gasoline fuel mixed with air is combusted by self-ignition inside a sufficiently compressed cylinder has attracted attention. The HCCI combustion is a mode in which the mixture gas combusts at a plurality of positions simultaneously, and thus has a higher combustion speed of the mixture gas than in SI combustion (spark-ignition combustion) which is adopted for general gasoline engines. Therefore, the HCCI combustion is said to be significantly advantageous in terms of thermal efficiency. However, the HCCI combustion has issues such as a combustion start timing of the mixture gas (a timing that the mixture gas self-ignites) greatly varying due to an external factor (e.g., atmospheric temperature) and a control during a transient operation in which an engine load sharply changes is difficult.
Therefore, instead of combusting the entire mixture gas by self-ignition, it is proposed to combust a portion of the mixture gas by spark-ignition using a spark plug. That is, after forcibly combusting a portion of the mixture gas through flame propagation caused by spark-ignition (SI combustion), the remaining mixture gas is combusted by self-ignition (CI combustion). Hereinafter, such combustion is referred to as “partial compression-ignition combustion.”
For example, JP2009-108778A discloses an engine adopting a similar concept to the partial compression-ignition combustion. This engine causes flame propagation combustion by spark-igniting a stratified mixture gas which is formed around a spark plug by a supplementary fuel injection, and then performs a main fuel injection inside a cylinder warmed up by an effect of the flame propagation combustion, so as to combust through self-ignition the fuel injected in the main fuel injection.
Here, in engines adopting the partial compression-ignition combustion, since the CI combustion in which the combustion speed of the mixture gas is high (thus, a pressure increase rate easily rises) is performed at least partially, combustion noise increases more easily compared to normal gasoline engines which only perform the SI combustion (spark-ignition combustion). Therefore, during the partial compression-ignition combustion, it is desirable to adjust a given control amount regarding combustion so that the combustion noise does not increase excessively (i.e., the ratio of the CI combustion does not increase excessively). However, when an upper limit of a combustion noise index value as a reference for adjusting the control amount is uniformly set disregarding the state of the engine or the vehicle, an adjustment in the control amount, which deviates from the combustion noise actually perceived by a passenger, is performed. Thus, the ratio of the CI combustion may be reduced unnecessarily or not reduced when required and harsh combustion noise may be transmitted to the passenger.